ppc4xx: Remove duplicated is_pci_host() functions

This patch introduces a weak default function for is_pci_host(),
returning 1. This is the default behaviour, since most boards only
implement PCI host functionality. This weak default can be overridden
by a board specific version if needed.

Signed-off-by: Stefan Roese <sr@denx.de>
diff --git a/board/esd/pmc440/pmc440.c b/board/esd/pmc440/pmc440.c
index ec92552..10901b4 100644
--- a/board/esd/pmc440/pmc440.c
+++ b/board/esd/pmc440/pmc440.c
@@ -712,7 +712,7 @@
 }
 
 /*
- * is_pci_host
+ * Override weak is_pci_host()
  *
  * This routine is called to determine if a pci scan should be
  * performed. With various hardware environments (especially cPCI and